Preheat your oven to 375°F (190°C) to ensure it's at the right temperature by the time your ingredients are prepared.

Rinse the zucchinis under running water and pat them dry with a kitchen towel. Cut each zucchini in half lengthwise to create 8 zucchini boats. Using a melon baller or spoon, scoop out the flesh, leaving about a 1/4-inch-thick shell, and set the scooped flesh aside. Brush the zucchinis with 1 tablespoon of olive oil and place them on a baking tray lined with parchment paper.

Heat the remaining olive oil in a skillet over medium-high heat. Add the chopped onion and garlic, sautéing them until the onion turns translucent. Add the ground beef to the pan, breaking it up with a spoon. Cook until the beef is no longer pink, then stir in the diced tomatoes, zucchini flesh, Italian seasoning, salt, and pepper. Let the mixture simmer for 5-7 minutes, allowing the flavors to meld.

Carefully spoon the beef mixture into each zucchini boat, stuffing them generously. Distribute the filling evenly among all the zucchini halves for balanced flavors in each bite.

Place the baking tray with stuffed zucchinis in the preheated oven. Bake for 20-25 minutes, or until the zucchinis are tender-crisp and the filling is heated through. Sprinkle shredded mozzarella over the top in the last 5 minutes of baking, allowing it to melt and form a delicious golden crust.